Severely disabled infants, such as infants with cerebral palsy (CP), frequently grow up to become passive children with limited or nonexistent speech, even when the infant possesses apparently normal cognitive skills. The following factors have been proposed to explain this passivity: limited oral motor control and a consequent diminished repertoire of speech sounds; limited ability to control its physical environment by manual manipulation of objects; and consequent limited opportunity to engage caregivers in mutually enjoyable interaction.
The incidence of infants with CP is estimated to be a least one per thousand live births. Some CP infants are likely to suffer from diminished control of the vocal tract and the respiratory system upon which speech depends. These physically impaired infants are commonly described by their parents as "quiet babies". Without a means by which a CP infant can vocalize, the cognitive and emotional development of the infant is at risk.
Babbling in infancy has been studied, and it has been found that an infant progresses through an identifiable sequence of developmental stages characterized by babbles of increasing syllabic structure and segmental contrastivity. The infant's progression through these babbling stages is assumed to be partly dependent on maturational changes in the configuration of the infant's vocal tract, and partly upon changes in motor control. Babbling has not been found to be closely related to cognitive level. Of particular interest is the finding that deaf infants use manual babbles, i.e., repetitive movements similar to the canonical babbles used by infants learning speech. This finding indicates that there is an innate capacity to practice the motor movements for babbling, regardless of whether the infant is learning to interact via speech, manual signing, or other form of non-vocal communication, such as device-assisted communication. Thus, babbling appears to be a form of exploration and rehearsal of the particular communication mode that the infant is learning which is necessary for later phonological development. Furthermore, feedback has been found to be important to an infant's progression through the various stages of babbling. For example, an infant is reinforced by the sound of his or her own voice.
The social reinforcement that the infant receives in the second half of its first year appears critical to the development of a vocalic repertoire, and to an understanding of the "rules of conversation" that must be developed to achieve meaningful communication with parents or other caregivers. By vocalizing and using manual gestures and changes of facial expression, infants can elicit responses from caregivers. By responding to such communicative initiatives, parents or other caregivers reinforce these activities. Physically disabled infants are unable to control the motor systems upon which speech is dependent, or the manual systems for playing with objects or making gestures. Due to motor impairment, they may also show little facial affect. Consequently, they lack the means for providing consistent signals to their parents or other caregivers that are available to normal infants. As a result, caregivers are unable to discern patterns of behavior to which they can attach meaning and respond accordingly. Ultimately, physically disabled infants are likely to grow up passive and with a diminished motivation to learn.
By exploring their environment, normal infants gradually develop the concept that an action brings about a consequence. The infant's early attempts at vocalization may be seen as an exploration of cause and effect using the vocal mechanism. Actions such as banging and shaking are developed and reinforced by interesting consequences. Also, toys such as rattles and noise-makers are designed to help the child in his or her explorations of cause and effect relationships. However, physically disabled infants are frequently limited in their ability to explore the environment and to vocalize, resulting in delayed or attenuated development.